Parametric amplification is a well known technique for increasing the Q of electronic oscillators whereby the stiffness of the system (the capacitance of an electrical oscillator or the elastic constant for a mechanical oscillator) is modulated at twice the natural resonant frequency. This overtone modulation can be shown to significantly increase the amplitude of the forced oscillator resonance, thereby effectively increasing the Q. A limitation on this technique for low frequency oscillators is the voltages of the resonating and the parametric amplification circuits are inversely proportional to frequency. Several groups have recently demonstrated this effect for MEMS-based Si resonators using thermally (laser) induced changes in the elastic modulus or specially designed springs near the active resonator. However, no attempt has been reported to apply this technique to piezoelectric shear-mode quartz resonators.
The prior art does not use parametric amplification at twice the resonant frequency for a quartz oscillator. Moreover, quartz oscillators in the 5-10 MHz range have traditionally been manufactured as discrete HF devices packaged in large ceramic packages, where the ability to modulate the elastic modulus with low drive voltages has been limited because the voltages needed for parametric amplification are often larger than the saturation values for small low noise circuits. This limits the level of integration achievable. Dransfeld, in Elastic Wave Parametric Amplifier U.S. Pat. No. 3,012,204, describes parametric amplification in quartz but not as an oscillator nor is the pump frequency twice the resonator frequency. Seidel, in Frequency Stabilized Oscillator U.S. Pat. No. 3,401,354, describes paralleling several discrete crystal oscillators of distinct frequencies and exploiting the parametric amplification of each oscillator by pumping each with a composite signal. Kano, in Parametric Acoustic Surface Wave U.S. Pat. No. 3,816,753, describes an oscillator based on counter propagating surface acoustic waves instead of the natural resonance of the crystal. Finally, Momosaki in Coupled Mode Tuning Fork Type Quartz Crystal Vibrator U.S. Pat. No. 4,320,320 couples one tine in a flexural mode at one frequency with another tine in a torsional mode at a frequency not an integer multiple of the first frequency.
There is a continuing need for VHF and UHF oscillators that operate with voltages that are compatible with CMOS electronics, are thermally stable and generate low noise.